1. Field of the Invention
The present invention relates to methods of inserting tubing into a well bore and to well head apparatus for performing such methods and is useful for the insertion of continuous tubing, e.g. of plastic, and of coupled tubing sections into live well bores.
2. Description of the Related Art
One of the differences between running coil tubing and coupled tubing into a well head is typically the way the well head is configured. Because it is usually predetermined that coil tubing is going to be installed into the well instead of coupled tubing, a full opening closure is installed at the surface to the top of the casing of the well bore before the well is perforated. The closure installation is generally consistent with coupled tubing live well installations, with the difference typically but not always being the type of closure. The closure may vary in size and configuration from a standard valve to any suitable well control device, e.g. a blow out preventor. The closure is dependent on the well application, with a typical prerequisite being full opening consistent with the inside diameter of the well casing or the largest size of any tool or mechanism that may be installed within the well head assembly, e.g. the tubing hanger assembly.
Once the well has been perforated, the full opening closure is closed so as to maintain the well pressure below the closure. A well head which is designed to hang or suspend coil tubing may be installed on top of the casing, after or prior to perforating the well, and below the full opening closure, once again depending on the application, the closure therefore being located above or below the wellhead. The well head is generally much the same as any conventional well head, with typical outlets and valves on opposing sides to allow access to the annulus (i.e. the space between the tubing and the casing) as may be necessary to deal with any particular well. The well head may also have a sealing device incorporated into the body of the head to allow movement of the tubing while maintaining well bore pressure.
Most coil tubing installed into wells today is steel and is injected into the well with a hydraulically activated injector head that has two opposed rolling surface areas that effectively push the tubing into the well from above the well head, using friction to ensure control and movement of the tubing into the well bore and thereby exerting compressive forces on the tubing. This process is very expensive because of the cost of the steel tubing and also because it requires the use of elaborate and therefore expensive equipment.
Plastic tubing, which is very inexpensive compared to steel tubing, has been inserted into wells using the same apparatus, but is limited to use with low well bore pressures because of the risk of buckling and collapsing in response to the compressive forces acting on the plastic tubing.
Another factor with the use of coil tubing in well applications is the difficulties encountered with depth measurement relating to the leading end of the tubing. The tendency for coil tubing to remain in a semi-coiled state in the well bore because of residual memory, resulting from the tubing being installed from a round spool, and also surface contact friction encountered as the tubing lays against the walls of the well casing increase friction, thereby making insertion into the well bore more difficult, increasing the potential for buckling and making depth measurement very inaccurate.